1. Field of Invention
The invention relates to a process for measuring particle concentrations in an aerosol. The invention also relates to an apparatus that implements that process. The process and the apparatus can be utilized for example in measuring particle concentrations in an exhaust system of a combustion engine.
2. Description of the State of the Art
For various reasons, it is important to be able to measure fine particles, especially the ones born in combustion engines. The measurement activities are driven by increasing concern about the potential health effects of fine particles, and these health concerns have led to limits being imposed on fine particle emissions. In the future, these limits on particle emissions will continuously become tighter. Another need for fine particle measurements arises from the development of combustion engines and from real-time monitoring of combustion engines, especially diesel engines. Still another need for fine particle measurements arises from the increasing production and use of nano-sized particles in industrial processes, the particles typically having a diameter of less than 100 nm. Both the monitoring of these industrial processes and occupational health and safety issues require reliable fine particle measurement.
Diesel engines exhaust particles in three different size ranges: nuclei-sized particles have a diameter of less than approximately 50 nm, accumulation-sized particles have diameters between 50 nm and 1 .mu.m, and coarse-sized particles have a diameter of greater than 1 .mu.m. A majority of the diesel engine exhaust particles are created after the exhaust gases escape from the exhaust pipe and these particles typically are nuclei-sized.
A particle trap is typically installed to the exhaust pipe of a combustion engine to keep the fine particle concentrations below the exhaust limits. The trap requires frequent regeneration and the trapped particles are combusted by increasing the temperature of the trap and simultaneously feeding excess air to the trap, so that the carbon containing particles are burnt.
The patent publication F1 118278 B, Dekati Oy, 25 Dec. 2004, relates to a method and a sensor device for determining particle emissions from exhaust gases of a combustion engine substantially during the use in an exhaust pipe system or a corresponding exhaust gas duct, in which method emitted particles contained in the exhaust gases are charged and the particle emissions are determined by measuring the electric charge carried by the emitted particles in said exhaust gas duct. According to the invention, the emitted particles are charged by varying the way of charging or the charging power over time in such a manner that, as a result of said charging, emitted particles brought into at least two different electrical charge states are present, wherein the charge of the emitted particles is further determined as a difference value/values measured from the emitted particles brought into said at least two different electrical charge states. The problem with the described method is that the particles are charged by a charger placed inside the exhaust gas duct, where the charger is easily soiled, shortening the lifespan and reliability of the charger. Varying particle concentration and constant ion generation cause problems in maintaining constant particle charging. The formation of fine particles in the exhaust duct is a complicated process and making measurements in an environment with a greatly varying mass flow is very difficult. Different fuels and different lubricants affect the particle concentrations and particle properties in the exhaust gas duct, as described e.g. in Heejung Jung, et al., The Influence of Engine Lubricating Oil on Diesel Nanoparticle Emissions and Kinetics of Oxidation, SAE International 2003-01-3179, 2003.
The publication of Francisco J. Romay, et al., A Sonic Jet Corona Ionizer for Electrostatic Discharge and Aerosol Neutralization, Aerosol Science and Technology, Vol. 20 (1994), pp. 31-41, describes the design of a bipolar corona ionizer using a pair of sonic jet ionizers of different polarities. The ionizer is characterized in terms of ion output and particle generation for several electrodes, orifice plate materials, and ionizer operating conditions. The sonic jet ionizer appears to generate more particles than typical free corona ionizers. This might be due to a stronger and more reactive corona discharge. The use of silicon electrodes in the sonic jet ionizer resulted in unacceptable levels of particle formation. Lower particle generation could be achieved by using tungsten carbide electrodes and molybdenum orifice plates.
Patent publication U.S. Pat. No. 6,544,484, TSI Inc., 8 Apr. 2003, describes a system for analyzing aerosols. The system incorporates a corona discharge ion generator with a positively or negatively charged corona discharge needle formed of platinum or a platinum alloy. A high speed (40-210 meter per second) air flow sweeps the ions away from the corona discharge, and propels the ions into a mixing chamber in a turbulent jet that encounters an aerosol that is also provided to the mixing chamber. In one version of the ion generator, the ions are carried into the mixing chamber through an orifice formed in a positively or negatively biased plate. In another alternative, the aerosol droplets are electrostatically generated, and propelled into the mixing chamber as an aerosol jet that opposes the ion jet to enhance mixing of the charged droplets and the ions. In this version, the droplets are advantageously neutralized to leave predominantly singly charged positive and negative particles. The problem with this system is that the flow ratio of the aerosol to the clean air may change during the measurement, for example due to contamination of the filter. This form of mixing ionized air with the aerosol by opposed jets is difficult because the set-up is very sensitive to jet properties like jet direction and jet velocity, and thus the jets tend to be unstable. Ion losses is such set-up are very high, typically more than 99% and even higher than 99.9%
Patent publication U.S. Pat. No. 3,413,545, Regents of the University of Minnesota, 26 Nov. 1968, describes an electric aerosol particle-counting and size-distribution system for particles in the 0.01 to 2 micron size range. An aerosol chamber unit having a gas ionizing device and a diffusion chamber imparts a unipolar charge on aerosol particles in proportion to the size of the particles. The charged particles are delivered to a mobility analyzer having a housing with an elongated chamber. A particle-collecting electrode projects axially into the chamber above a current collector and a sensor filter connected to an electrometer. Collecting voltages up to 30 kV are used in the system. The mixing in the system is inefficient and thus the ion losses are probably very high, typically more than 99.9%.
Patent application US 2006/0144124 A1, Takeshi Kusaka, et al., 6 Jul. 2006, describes a soluble organic fraction, SOF, measuring system that can continuously measure SOF and a soot measuring system that can continuously measure soot. The two are connected by an exhaust gas line. The soot measuring system comprises a diluter that selectively dilutes either one of the exhaust gas and a standard gas whose hydrocarbon concentration is known with diluent gas, and extrudes it. A dilution ratio adjusting device can adjust a dilution ratio of the diluter. A soot detector continuously detects soot in the exhaust gas or the standard gas diluted by the diluter. The SOF measuring system can be connected with the diluter so that an exhaust gas analyzer can measure the hydrocarbon concentration in the standard gas diluted by the diluter. The publication does not mention charging the diluting air.
The problems with the prior art systems are: charger soiling, poor mixing of ionized air and aerosol, varying sample flow, and high ion losses. All these problems make the prior-art systems unstable, especially for on-line measurement of combustion engine exhaust gas particles. The current systems are also large and cannot be used in monitoring the particle emissions from diesel vehicles. The large mixing chamber in some prior-art systems also slows down the response time of the measurements.